One of the methods currently used for the in-flight refueling of aircraft comprises the use of a boom provided with an internal section with capacity for telescopic extension via which aircraft fuel is transferred by means of an internal line from the tanker aircraft to the receiving aircraft. The boom is connected to the tanker aircraft by means of a hinged system provided with two degrees of freedom and is equipped with a set of movable aerodynamic surfaces or fins which, by means of independent variations of the incidence of each of them relative to the incident air flow, enables the operator of the system to position the boom in the conditions of desired attitude. The limit of the attitude envelope attainable depends on the maximum capacity of said fins for generating forces that make it possible to balance the set of aerodynamic, inertial and gravitational effects relative to the linkage of the system.
Movement of the boom from one position of equilibrium in space to another is effected by using these fins, with their initial and final positions corresponding to those that balance the boom in both spatial conditions. Said movement is a movement of rotation about each of the two axes defined by the two degrees of freedom of the linkage (elevation-roll or elevation-azimuth). The dynamic characteristics of said system correspond to a modal description characterized by a very low damping ratio. Consequently this movement generated by a direct change of fins from that corresponding to equilibrium in one spatial position to another spatial position involves an oscillatory movement of the boom with very little damping.
To accomplish the precise and predictable operation of tracking of the refueling socket or receiver of the receiving aircraft, without running the risk of causing an impact between the recipient and the boom, it is necessary for the movement of the latter to be sufficiently damped.
For achieving this aim, the prior art employs electrical control systems (fly-by-wire), with functions for increasing stability and control, in such a way that the dynamics of movement, by means of said control system, has the desired characteristics for performing the task of tracking the receiver. An electrical flight control system comprises:                Elements for measuring movement and/or position of the platform to be controlled, called sensors.        A control element in the operator's cabin, generally a joystick, for controlling the movements required by the operator.        Actuating elements that generate the movement of the control elements of the platform, in this case the aerodynamic surfaces.        An on-board computer or calculating system with on-board software that includes algorithms which, when supplied with the orders generated by the operator, together with the measurements of movement and/or position of the equipment, generate a command for movement of the control elements, via their actuating systems.        
The algorithms implemented in the on-board computer are directed towards modification of the natural dynamics of the equipment to be controlled as well as provision of other functions such as a manoeuvre or position demand system.
In the case of the in-flight refueling boom system, the control algorithms must perform a basic function of increasing the damping of the basic equipment, from a very low value, close to zero, up to a value compatible with a rapid and predictable response without oscillations or exceeding of the final value demanded. This desired response is associated with appropriate operating conditions for performing the task without requiring significant compensation of the dynamics of the equipment by the operator.
The increase in damping is achieved by means of a command for movement of the aerodynamic fins so that they generate a load, producing an overall force on the boom that tends to oppose the angular movement of the latter. The damping effect is achieved by means of a fin movement of a magnitude related to the angular velocity (normally a proportional relation) and opposite in direction to this angular movement. The basic characteristics of the refueling system necessitate increased damping of the angular movements defined by both linkages (elevation-roll or elevation-azimuth).
Owing to the fact that in the design of the refueling probe the length of the probe predominates, as well as its lowest possible weight, the structural characteristics of the boom are characterized by particular modes with extremely low frequencies. These frequencies are close to the natural frequencies that define the movement of the boom as a rigid solid about the linkage and the frequency of control thereof used by the operator for controlling said movement. These flexible modes are also characterized by a very low damping ratio, exhibiting a very characteristic resonance frequency with a high level of amplification of the response if excited at said natural frequency.
The flexible modes of the probe are excited when subjected to external loads, such as the forces generated by the fins in the process for controlling the position of the boom. The excitation through movement of the aerodynamic surfaces has two origins: one due to the change in aerodynamic load on changing its angle of incidence, the other due to the inertial loads caused by rotation about its axis of rotation. In the case of the in-flight refueling boom, the effect of generation of aerodynamic load is dominant.
The elements of the flight control system that detect the movement of the boom measure the movement of the section of the boom where the measuring elements are located, detecting both the movement as a rigid solid, the control object thereof, and the movement associated with the flexible vibration of the structure.
Therefore the control algorithms are fed with the rigid and elastic movements, which are converted by these algorithms into a demand for movement of the aerodynamic fins. By their movement, these in their turn generate new forces which in addition to acting on the movement of the boom as a rigid solid, once again excite the structural modes. This combination of rigid and elastic movements is fed again to the control system, and a coupling effect is produced. The effect of coupling of the flexible movement of the structure with a control function as a rigid solid is called “structural coupling”.
The excitation of the structure through movement of the aerodynamic fins decreases as the frequency increases, owing to the natural attenuation of the actuating system, as well as the intrinsic capacity for generation of aerodynamic force.
This effect of feedback of the movement must be such that the resultant dynamics are stable, and must moreover guarantee a margin relative to the condition of instability of the complete system, formed by the equipment to be controlled plus its control system. In the qualification of system airworthiness, minimum stability margins are specified both in the control of the rigid modes, and in the stability of the flexible modes.
In the phenomenon of structural coupling, the closeness of the rigid and the flexible control frequencies is critical, therefore the first flexible modes are of low frequency and are relevant to the phenomenon. The dominant flexible mode will be regarded as that having greatest influence on the problem of structural coupling, i.e. by its frequency, by a low damping ratio, by excitation due to the deflection of aerodynamic fins or by its detection by means of the elements for measuring movement or position.
A first measure for reducing this phenomenon is to locate the measuring systems in positions that are not altered by the movement due to the flexible modes relevant to the phenomenon, or even to locate the control surfaces at a point of less excitation of the structure. Whereas the location of the elements for measuring movement can at times have some flexibility during design, the positioning of the control surfaces has to satisfy the capacity for control and balancing of the equipment. In any case, positioning of the measuring elements in a position that is not altered by the movement due to the relevant flexible modes is not a robust solution owing to the change of these modes with possible changes in configuration of the equipment, as is the case with the movement of extension of the telescopic tube in the case of the refueling boom.
In the case of the boom, the aerodynamic fins must be positioned to be sufficiently remote from the linkage to produce a balancing moment about the linkage by means of a force that is as small as possible. The measuring elements can be located in the sections that interfere as little as possible with the transverse section of the boom, so that the aerodynamic drag does not increase significantly. These can be positioned at the root of the boom and in the section with the fins.
The usual techniques for avoiding the phenomenon of structural coupling are based on performing signal filtering, whether those obtained via the measuring systems, and the signals for command of fin movement, in such a way that there is no change in content of the signal with respect to measurement of rigid movement. The negative effect of said filtering is that the more we wish to attenuate the content of the flexible signal at high frequency, the more phase lag is induced in the signal in the range of control frequencies as a rigid solid. This phase lag means that the maximum damping ratio that could be obtained by means of feedback is limited. The greater the phase lag, the lower will be the maximum damping ratio attainable by means of feedback, and therefore the handling qualities that can be achieved will be poorer.
In the case of the in-flight refueling probe, the closeness of the flexible frequencies and the control frequencies means that the usual method of filtering cannot be used for simultaneously achieving the objectives of guaranteeing the required stability margins and providing adequate handling characteristics for performing the task of tracking of the receiver. The present invention aims to correct this shortcoming.